1. Field of the Invention
The present invention relates to a birefringent optical film, an elliptically polarizing plate using the optical film, and a liquid crystal display using the elliptically polarizing plate having the birefringent film.
2. Description of Related Art
Conventionally, birefringent films for optical compensation, which are used in various liquid crystal displays, have been manufactured by film-stretching techniques. Regarding the stretching, for example, JP 3(1991)-33719 A discloses roller tensile stretching, roller press stretching, and tenter uniaxial stretching. JP 3(1991)-24502 A discloses biaxial stretching under a condition of providing anisotropy. Regarding other methods of manufacturing birefringent films, for example, JP 8(1996)-511812 A discloses a method of processing a soluble polyimide into a film so as to provide negative uniaxiality.
By using the above-described or other techniques of film stretching, optical properties such as nx≧ny>nz can be applied to the film (nx and ny denote main refractive indices in the film plane, and nz denotes a refractive index in the thickness direction of the film). This birefringent film arranged between a driving cell and a polarizer can be used as a viewing angle compensation film for a liquid crystal cell, thereby widening a viewing angle of a liquid crystal display.
For example, a biaxial birefringent film can be used as an optical compensation film for obtaining a wide viewing angle in a vertical alignment (VA) mode liquid crystal display with a multi-domain alignment. This biaxial birefringent film can be obtained in general by stretching a polymer film. Specifically for example, it can be obtained by stretching in two directions (x- and y- directions) on a film surface, or a fixed-end uniaxial stretching, i.e., stretching the film in a transverse direction while the film is fixed at one end in the longitudinal direction (e.g., a transverse stretching with a tenter).
In general, the three-dimensional refractive indices nx, ny and nz in the biaxial birefringent film can be controlled. Particularly, an in-plane retardation value Δnd and a retardation value Rth in a thickness direction can be controlled. The Δnd and Rth are represented by the following equations, where d denotes a thickness of a birefringent film.Δnd=(nx−ny)·dRth=(nx−nz)·d
The above-described Δnd and Rth can be controlled, for example, by the stretching temperature, the stretching rate in the x- and y-directions or the like. Specifically for example, Δnd can be controlled by a ratio of the x-direction stretching to the y-direction stretching, while Rth can be controlled by stretching degrees in the x- and y-directions. Formation of Rth is especially important for a biaxial birefringent film. Particularly, compensation of the birefringence of a VA mode liquid crystal depends largely on the Rth.
In forming the Rth, when the polymer material (e.g., polycarbonate) of the birefringent film has a high stretching orientation, a desired Rth can be obtained without increasing considerably the stretching degrees in the x- and y-directions. However, since the material will increase the photoelastic coefficient as well, the refractive indices will be varied easily when slight external force due to dimensional change of a polarizing plate or the like is applied. In addition to that, in a liquid crystal display assembled with this film, the contrast deteriorates partially under a severe condition caused by application of heat or moisture, and this causes considerable damage in the in-plane uniformity.
A birefringent film made of a material having a small photoelastic coefficient (e.g., a polynorbornene-based material) experiences substantially no changes in the birefringence even when applied with external force. A liquid crystal display assembled with this film will not lose its uniformity in the display even under the above-mentioned severe condition. However, since the material has a low stretching orientation, a single film may not provide sufficient Rth. For obtaining a desired Rth with the film, it is required to increase the stretching rates in the x- and y-directions and also using a plurality of retardation plates. This will cause degradation in accuracy of Δnd and Rth, degradation of the optical axis accuracy due to a bowing phenomenon, increase in panel thickness, and a cost rising.
That is, any of the conventional techniques has a difficulty in obtaining a birefringent film providing both a high orientation for obtaining easily a desired retardation value and a small photoelastic coefficient for reducing irregularities in the contrast during a durability test performed by applying heat and/or moisture.